Should be a pretty simple program.
The first sensor when tripped should store a starttime variable value using the millis() command and the second sensor should store a stoptime variable value also using the millis() command. Then you just have to subtract the starttime from the stoptime and you will have the elapsed time in milliseconds. You can then scale that and send it to your display.
Here is the info on millis()
Make sense?
Lefty